Diffusion helps amoeba, unicellular protists to survive because it eliminates their wastes and supplies them with useful substances needed for growth. Whereas, multicellular organisms like humans have developed complex organ systems specific for their function and metabolism.
Hence diffusion is not the process that can fulfill all the requirements of the organism. Complete answer:
Diffusion is defined as the movement of molecules (liquids or gases) from a region of higher concentration to a region of lower concentration.
It is an important process for living things. Molecule moves in and out of the cell through diffusion.
Unicellular organisms like amoeba use diffusion to obtain oxygen and expel waste gas (carbon dioxide) and are sufficient enough for their survival.
But in the case of complex multicellular organisms like humans and animals that have cells clustered in the form of tissues and organs, diffusion alone cannot help them sustain and survive in the environment.
In the case of humans who have developed various complex organ systems, oxygen requirements through the multicellular complex organ system, diffusion alone cannot serve the purpose.
Thus, all life processes such as respiration, digestion, excretion, and all life processes in amoeba can be done through diffusion. Note:
Amoeba, being a unicellular organism, obtains oxygen and expels waste gas (carbon dioxide) by diffusion.
Thus, in amoeba diffusion is used as the method of respiration.
In human beings, diffusion of gas from the head to the feet may take up to 3 years.